The present invention relates to ink jet printers and, more particularly, to an ink jet printer and method of printer shut down in which ink is maintained within the print head during periods of time in which the printer is shut down.
A number of different types of ink supply systems have been utilized in the past for supplying ink under pressure to the print head of an ink jet printer. Typically, the print head defines a fluid reservoir to which ink is supplied under pressure and at least one orifice communicating with the reservoir. Ink flows through the orifice and forms a fluid filament. Mechanical disturbances are applied to the fluid filament, as for example by means of a piezoelectric transducer, to stimulate the filament to break up into a jet drop stream. As drops are formed from the fluid filament, the drops are selectively charged and, thereafter, are deflected by an electrostatic field such that they are separated into print and catch trajectories. The drops in the print trajectories strike a print receiving medium, such as a paper web, while the drops in the catch trajectories are directed to a drop catcher, which ingests the drops and returns them to the ink supply system for reuse. One such prior art printer is shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis.
One type of ink jet printer fluid supply system is shown in U.S. Pat. No. 3,761,953, issued Sept. 25, 1973, to Helgeson. The Helgeson system includes a fluid recirculation path from a fluid pump to a pressure regulation tank and back to the pump. Fluid is withdrawn from the presssure regulation tank and supplied to the print head. Ink in the recirculation path is replenished from a supply tank.
A significant problem encountered with ink jet printers is the difficulty of providing a start up of the printer in which the jet drop streams are formed without wetting other printer components. As the flow of ink through the print head orifices begins, the jet drop streams initially established tend to be somewhat unstable, both in trajectory and in drop size. This instability may also reappear at shut down of the printer as the fluid flow through the orifices is terminated.
U.S. Pat. No. 4,042,937, issued Aug. 16, 1975, to Perry et al, discloses an ink supply system in which sequencing of purging, start up, print operation, and shut down of the printer are controlled by a pair of solenoid-actuated valves connected in the inlet and outlet lines of the print head. The inlet valve is connected between a pump and the print head, while the outlet valve is connected between the print head and the supply tank which provides ink to the pump. Start up is accomplished by filling the print head with ink, closing the inlet valve to permit pressure to build behind the inlet valve to a level significantly greater than that required for operation and, thereafter, opening the inlet valve. At shut down, the inlet valve is closed while the outlet valve is held open, thus creating a negative pressure in the head. The pump is then turned off.
Ink is maintained within the print head during the period of time in which the printer is shut down. Perry et al suggests that it may be desirable to purge the print head of air bubbles subsequent to shut down to prevent drying of ink inside the print head. Nevertheless, drying of ink and precipitation of particulate contaminants may occur within the print head during protracted shut down periods, with the result that the print head nozzles or orifices may become clogged. Additionally, there is the possibility that ink may weep through the nozzles during periods of shut down, producing undesirable wetting of various printer elements.
Other types of ink jet printers have included a provision for removing all ink from the print head reservoir during periods of printer shut down in order to minimize clogging of the print head orifices. This necessarily complicates start up and shut down of the printer, however. U.S. Pat. No. 3,970,222, issued July 20, 1976, to Duffield, discloses an ink jet printer start up method in which ink is supplied under pressure to the initially dry print head reservoir to compress the air in the reservoir. This, in turn, raises the pressure of the ink. Compression of the air continues until the ink reaches the first orifice in a row of orifices, at which time the pressure within the print head is in excess of the required start up pressure. Ink flows through the first orifice and, in succession, through each of the other orifices of the print head.
U.S. Pat. No. 3,891,121, issued June 24, 1975, to Stoneburner, discloses a start up method in which the print head manifold, initially dry, is pre-pressurized with air and a flushing liquid before supplying ink to the manifold. At shut down of the printer, the flow of ink to the print head manifold is replaced with a flow of flushing fluid. The flow of flushing fluid is then terminated and, simultaneously, an evacuation line leading to a low pressure source is opened, removing fluid from the manifold. The manifold is thereafter maintained in a dry condition until start up of the printer is subsequently initiated.
In order to avoid the difficulties encountered in start up of a print head of the type from which ink is removed during shut down periods, while at the same time eliminating the possibility of ink drying in the print head nozzles and clogging the nozzles, as may occur with printers of the type in which ink is maintained within the print head during shut down periods, U.S. Pat. No. 3,839,721, issued Oct. 1, 1974, to Chen et al discloses a printer arrangement having a liquid filled container which is movable with respect to the jet nozzles. The container, filled with water or water containing detergent, submerges the print head nozzles during shut down periods and prevent drying of ink within the nozzles. This permits ink to be maintained within the print head mainfold during periods of printer shut down. In an alternative embodiment, the nozzles are submerged in a mist or vapor which prevents ink drying. The liquid filled container for submerging the nozzles and the mechanical arrangement for moving the container add significantly to the size and cost of the printer. Additionally, contaminants in the ink within the print head may settle during periods of printer shut down, causing the nozzles to become clogged.
Accordingly, it is seen that there is a need for an ink jet printer of simple, reliable design, capable of maintaining ink within the printer print head during shut down periods without the ink drying and without particle deposition which could cause the print head orifices to become clogged. Such a printer should also be configured to prevent weeping of ink through the print head orifices during shut down periods.